The present invention relates to a highly efficient method of preparing secondary aminoisobutylalkoxysilanes via noble metal-catalyzed hydrosilation reactions between secondary methallylamines and hydridoalkoxysilanes.
Secondary aminoisobutylalkoxysilanes have long been accessible by various chemical approaches, and have recently demonstrated commercially useful performance in polyurethane sealants by providing crosslinking sites for alkoxysilane-functional polyurethanes (see EP 676,403 and U.S. Pat. No. 6,197,912, incorporated herein by reference). The preparation of such silanes has been achieved with some degree of complexity, however.
The preparation of Me(MeO)2SiCH2CHMeCH2NHMe is reported (Journal of Organic Chemistry, vol. 36, 3120(1971)) via a series of reactions including the hydrosilation of methallyl chloride with MeSiHCl2, reaction of that product with a large excess of MeNH2, and reaction of the cyclic silazane so formed with MeOH. A similar trialkoxysilane version was made by a slightly different sequence of reactions as disclosed in Brit. 994,044, whereby methallyl chloride is hydrosilated with trichlorosilane, followed by reaction of that product with ethanol to form (EtO)3SiCH2CHMeCH2Cl, and reaction of that product with excess MeNH2 to form (EtO)3SiCH2CHMeCH2NHMe. These two processes both involve three steps, namely hydrosilation, esterification, and amination, plus a final purification, as by distillation, such that these processes are not commercially or economically attractive. While bis(alkoxysilylisobutyl)amines, the putative products of hydrosilations of dimethallylamine with hydridoalkoxysilanes or hydridoalkylalkoxysilanes, would be very difficult to prepare by the former process involving cyclic silazane intermediates, they are reported as low yield by-products of the latter process, involving reactions of chloroisobutylalkoxysilanes with ammonia. A similar molecule is also reported as a by-product from the reduction of 2-cyanopropyltriethoxysilane (U.S. Pat. No. 2,930,809), and as a crosslinked aminosiloxane resin raw material, prepared by reaction of chloroisobutyltriethoxysilane with ammonia (U.S. Pat. Nos. 4,410,669 and 4,455,415). The product of EP 676,403, namely Me(MeO)2SiCH2CHMeCH2NHC6H5, was prepared by reaction of excess aniline with Me(MeO)2SiCH2CHMeCH2Cl and involves the aforementioned three steps plus distillation.
There is thus a continuing need in the adhesives and sealants art for secondary aminoisobutylalkoxysilanes, including secondary bis(alkoxysilylisobutyl)amines which can be prepared in high yields and high purities by processes which are efficient in terms of both reaction time and output per unit volume of equipment used, which generate minimal amounts of waste and by-products, and which are simple in terms of number of process steps and number of raw materials, additives, or promoters which need to be charged to said equipment.
Methallylamine is disclosed and/or claimed in a number of patents involving hydrosilation of allylic amines using hydridosiloxanes, but there is no working example of such a reaction, and there has been no suggestion to hydrosilate methallylamine using a hydridosilane. In particular, there is a disclosure of hydrosilation of a secondary methallylamine with a hydridosiloxane (U.S. Pat. No. 5,486,634) and limited art on hydrosilations of tertiary methallylamines, which would yield products of no use in capping polyurethanes. There is also no working example of hydrosilation of dimethallylamine, although that amine is disclosed in at least one hydrosilation patent also involving hydridosiloxanes (U.S. Pat. No. 5,840,951).
Historically, hydrosilations of allylic amines have been notoriously unsuccessful. Allylamine is specifically excluded in an early general patent on hydrosilation (U.S. Pat. No. 2,970,150), with hydrosilation products being prepared by capping the allylamine with trimethylsilyl groups, hydrosilating the allyl group, and removing the trimethylsilyl groups (see Journal of Organic Chemistry, Vol. 24, 119 (1959)). Alternatively, aminopropylalkoxysilanes have been prepared by reactions of chloropropylalkoxysilanes with large excesses of ammonia or primary amines, yielding the respective primary or secondary aminopropylalkoxysilanes. These routes suffer from low yields per unit volume of equipment used, high levels of waste or excess raw materials, and the formation of large amounts of difficult-to-handle solid hydrochloride salts. Aminopropylalkoxysilanes have also been prepared by reduction of cyanoethylalkoxysilanes, which are prepared by hydrosilation of acrylonitrile with chlorosilanes, followed by esterification with the appropriate alcohol. These are multi-step processes, followed by purification as by distillation.
A number of later developments have allowed the noble metal-catalyzed hydrosilations of allylamine with various SiH-containing reactants, although the reactions have been impractically slow and/or incomplete, unless run at a higher temperature, usually under pressure, in the presence of a hydrosilation promoter (U.S. Pat. No. 4,481,364). Further improvements in yields have been obtained with rhodium catalysts instead of platinum catalysts, with additives or promoters being necessary both for said improved yields and for providing products with lowered contents of undesired internal isomer adducts (U.S. Pat. Nos. 4,556,722; 4,888,436; 4,897,501; 4,921,988; 4,927,953).
The situation regarding diallylamine is further complicated by the formation of disproportionation by-products, which do not occur with allylamine itself (Zhur. Obshch. Khim., Vol. 44, 1484 (1974), in English as Journal of General Chemistry, USSR, Vol. 44, 1456(1974)).
Given this history there has heretofore been no reason to expect that secondary aminoisobutylalkoxysilanes could be prepared in high yield by a direct hydrosilation reaction of an alkoxyhydridosilane and a methallylamine compound.
The present invention provides a simple, highly efficient method for preparing secondary aminoisobutylalkoxysilanes by the noble metal-catalyzed hydrosilation of secondary methallylamines with hydridoalkoxysilanes or hydridoalkylalkoxysilanes. The reactions proceed in high yields and conversions to yield isomerically pure products in the absence of additives or promoters normally required for hydrosilations of allylic amines. The process involves neither modification of the secondary methallylamines nor use of added hydrosilation promoters or solvents. Ordinary noble metal catalysts can be used.